i. Field of the Invention
This invention relates to internal combustion engines and methods of operation thereof.
ii. Prior Art
In recent years, present internal combustion engines have been subjected to increasing demands to satisfy environmental and economic considerations of our modern society. While some improvements have been made, the combined objectives of improved fuel consumption and low emissions have formed a complex problem, since the many factors involved are largely conflicting, and generally the methods used to reduce emissions tend to increase engine fuel consumption which, in the light of the urgent need to conserve and wisely use our energy resources, is totally unacceptable.
Any form of liquid or gas fuel will for a given weight when completely burnt, release a fixed amount of heat. Present, well-developed engines possess combustion efficiencies which leave little scope for improvement in indicated thermal efficiency. However, substantial improvements can be obtained by ensuring that the fuel's heat energy is released early in the expansion, with suitable consideration of the mechanical aspects of conversion, and then taken through a large expansion ratio, at all times regardless of engine load, so that the bulk of the expansive energy can be converted to usable rotary torque. This process must also be achieved using lean fuel to air ratios as not only does thermal efficiency rise as the fuel's ratio is reduced, due to the lower flame temperature reducing direct heat loss, but it also stimulates complete burning; this lean operation also being vital to achieve a reduction in undesirable exhaust emissions.
In conventional petrol engines operating on the common Otto cycle these previous requirements can not be met. This is because such engines depend upon combustion, under all load conditions, of an air fuel ratio that can be readily ignited by the use of a spark plug. As air fuel mixtures can only be ignited by a spark if the ratio of air to fuel is close to that of a chemically correct mixture, means must be provided to ensure that, regardless of engine load, this mixture ratio is maintained. To achieve this, both the volume of air and fuel must be regulated in accordance with the desired engine load by the use of throttling.
When the amount of air taken in by the engine's displacement is restricted a number of undesirable effects arise. Firstly, under part load, which is how an automotive engine spends most of its time, the engine is unable to use an efficient expansion ratio due to the effects of throttling. The cost of this is significant, as the brake thermal efficiency rises dramatically through the compression ratio's range in spark ignited engine. Secondly, the engine is subjected to an additional negative load, as when throttled for light load it in effect works as a vacuum pump. Thirdly, the fuel and air mixture, being constantly close to stoichiometric proportions, always burns at almost the maximum obtainable temperature so it can not obtain the reduced direct heat loss benefits of a lower cycle temperature. Its direct heat losses are therefore correspondingly high while specific heat effects and disassociation of the combustion products further reduces efficiency. This common spark ignition method of operation, having at the best of times only just sufficient air to consume the fuel, results in the combustion being undesirably slowed down while it is seldom complete, with a portion of the fuel's carbon content being expelled as carbon monoxide.
If the diesel engine and the methods it uses are considered, it can be readily appreciated why the thermal efficiency of such an engine is higher than a petrol engine and why the overall specific fuel consumption of a diesel engine, when used in identical automotive application to a petrol engine, is almost 50% less. The diesel gains this significant improvement mainly by avoiding the need for throttling. Its load is controlled solely by reducing or increasing the fuel consumed. This fuel is burnt rapidly, and under light loads completely and at all times the heat so produced is expanded through a large and efficient expansion ratio. It can also take full advantage of a lower cycle temperature, and its efficiency drops little as the engine's load is reduced while the exhaust products normally contain little, if any, carbon monoxide.
Due to these factors the diesel engine exhibits more potential to meet current demands for reduced emissions and fuel consumption than any other present form of engine. However, these advantages are to a large extent sacrificed when a diesel engine is designed to fill the exacting requirements of automotive use and has to conform to the performance standards set by the petrol engine, which we are used to and expect in a vehicle.
The main disadvantage of the diesel engine is the high working pressures involved to make the cycle practical. It is subjected to pressure loading which can not be utilized without great attention to the strength, tolerances and materials used in its construction and, generally for a given power output, will cost twice as much to manufacture as a petrol engine. Even with the finest selection of alloys the engine components also weigh more and require greater bearing areas. Consequently, if automotive engine speeds are approached, the additional friction and dynamic forces increase the mechanical losses dramatically so that much of its potential fuel savings are lost, as high speed low load engine operation is the norm for automotive use.
The high peak pressures generated in a diesel engine also create shock waves that are transmitted through the engine castings, producing objectionable noise and torsional vibrations that are difficult to control, so the power flow is not as smooth as an equivalent petrol engine. In order to approach petrol engine speeds, a diesel engine must utilize some form of turbulent swirl chamber to increase the air speed to a level that will enable complete mixing and combustion of the fuel in the short time available. While this works well, and comparatively high engine speeds can be obtained, its use further degrades the diesel's potential fuel consumption; the agitation of the air takes place during the compression process and this compression must result in attainment of a high enough temperature to readily ignite the injected fuel. As the air is forced into the swirl chamber it gives up much of its compression heat, so to compensate for this loss a very high ratio of compression must be used. This action correspondingly increases the cycle pressure further; also this higher level of compression increases the density of the working fluid during combustion, as density of gas is a major factor in heat exchange; direct heat loss presents a problem.
Being forced to use such high compression to ensure combustion is, unfortunately, not accompanied by such a corresponding increase in efficiency as one would expect. As compression levels are increased a point is reached at which further increase is not worthwhile from a practical standpoint. Efficiency gains are very worthwhile up to about 12:1. After that they are dramatically and progressively smaller and with normal engine methods, impractical to obtain. The high speed automotive diesel is forced to use a very high level of compression solely to provide ignition heat, its level needing to be almost double that of the practical ideal of about 12:1.
The object of this invention is to demonstrate how the performance of any suitable engine, with these comparatively minor changes in construction and combustion methods, can be manipulated to produce the beneficial engine characteristics of a diesel engine without resorting to impractical compression levels to obtain ignition, and the corresponding high cycle pressures which form an unavoidable aspect of normal true diesel practice. With this method there can be provided, by split phased combustion, an approach which avoids the undesirable aspects of the diesel and the Otto cycle while fully utilizing the beneficial features of each cycle.